As is known, the most diffused technique for obtaining ethylene polymers or ethylene-based copolymers consists in passing the reactants through a reactor-container fitted optionally with a stirrer, or through a tubular reactor or also through reactors consisting of a combination of both, at pressures in general greater than 500 atm. and at temperatures comprised between 100.degree. and 400.degree. C., by using as initiator agents of the reaction substances generating free radicals under operational conditions or using other types of catalysts suited for producing ethylene polymers or copolymers.
In general, the tubular reactors used in the polymerization of ethylene consist of a high-pressure tube closed into a sleeve; through the annular interspace between sleeve and tube there flows a cooling fluid along the reaction zone in order to allow a heat exchange towards the outside inasmuch of this type of reaction is intensely exothermic, while it is heating in the preheating and starting zone.
In fact, the known tubular reactors are subdivided into different sections, depending on their length, the length being equal to many times the diameter of the high pressure tube, for instance, from 250 to 40,000 times. More particularly, these tubular reactors have always a zone for the preheating of the ethylene, this zone extending up to the point where the exothermic reaction is started, and a heat subtraction zone generally corresponding to the reaction zone which absorbs part of the heat developed by the reaction thereby hindering the formation of hot points where there may set in the decomposition reaction of the ethylene. A portion of heat is removed through the walls of the tube by the refrigerating fluid circulating in the outside sleeve, while the remaining part of the reactions heat is absorbed by the reaction mixture (ethylene that had not reacted and polymer), wherefore the temperature of the mixture itself tends always to rise.
The heat that develops is, in fact, hindered to flow outside because of the great thickness of the walls of the reactor that has to stand very high pressures, up to 2,000 atm. and more, developed not only by the reaction but also due to the effect of anomalous reactions of the ethylene such as for instance the thermal decomposition of the reactants due to the particularly high temperature.
Since it is therefore necessary to limit the temperature of the ethylene for the above-mentioned reasons, it follows that the maximum yield of the exothermic reaction is thus limited.
Various different methods have been suggested and used for the removal of the reaction heat. Of these, the most widely used consists in an intermediate feeding of fresh ethylene which must, however, be suitably proportioned so as not to reduce the reaction temperature to such an extent as to cause the interruption of the reaction itself.
Moreover, there still remains the problem of supplying all the preheating and starting heat that must be fed to the reactant at very high temperature because of the difficulty to have a proper heat exchange. There must also be kept in mind that both the heating as well as the cooling, for certain types of polymer, must always be achieved through the walls of the tubular reactor, inasmuch as the method of feeding in fresh gas provokes modifications in the quality of the type of polymer.
In any event, since the great thickness of the tube, as already explained, reduces the global coefficient of the thermal exchange, for instance, also by three times or more, it follows that whatever the refrigerating fluid used and whatever the geometry of the outside surface of the high-pressure tube, the result will change little with reference to the effects of the modification of the coefficient of global exchange.